Signal compression is the translating of a signal from a first form to a second form wherein the second form is typically more compact (either in terms of data storage volume or transmission bandwidth) and easier to handle. The second form is then used as a convenient representation of the first form. For example, suppose the water temperature of a lake is logged into a notebook every 5 minutes over the course of a year, generating thousands of pages of raw data. After the information is collected, however, a summary report is produced that contains the average water temperature calculated for each month. This summary report contains only twelve lines of data, one average temperature for each of the twelve months.
The summary report is a compressed version of the thousands of pages of raw data because the summary report can be used as a convenient representation of the raw data. The summary report has the advantage of occupying very little space (i.e. it has a small data storage volume) and can be transmitted from a source, such as a person, to a destination, such as a computer database, very quickly (i.e. it has a small transmission bandwidth).
Sound, too, can be compressed. An audio signal comprising spoken words (speech) comprises continuous waveforms that are constantly changing. The signal is compressed into a digital format by a process known as sampling. Sampling an audio signal involves measuring the amplitude of the analog waveform at discrete intervals in time, and assigning a digital (binary) value to the measured amplitude. This is called analog to digital conversion.
If the time intervals are sufficiently short, and the binary values provide for sufficient resolution, the audio signal can be successfully represented by a finite series of these binary values. There is no need to measure the amplitude of the analog waveform at every instant in time. One need only sample the analog audio signal at certain discrete intervals. In this manner, the continuous analog audio signal is compressed into a digital format that can then be manipulated and played back by an electronic device such as, for example, a computer or a personal digital recorder. In addition, audio signals can be further compressed, once in the digital format, to further reduce the data storage volume and transmission bandwidth to allow, for example, high quality audio signals to be quickly transmitted across even low bandwidth interlinks.